1. Field of the Invention
This invention relates in general to electromechanical filters and in particular to a novel electromechanical filter.
2. Description of the Prior Art
Mechanical filters have proved very advantageous for frequency selecting and filtering in transmission of electrical communication signals because of the high quality of the individual resonators which makes it possible to obtain with very small construction relative to electrical LC filter circuits superior results. Electromechanical filters also have very good temperature characteristics in that their frequency response varies very little if any with temperature changes.
Mechanical resonators can oscillate in various modes as, for example, the longitudinal mode, the torsional mode and the bending mode. The resonators must also be mechanically coupled together and it is necessary in constructing mechanical filters to suitably select the most desirable oscillation mode for the resonators as well as the proper mechanical coupling elements. The bending mode is a particular advantageous oscillation mode for the resonators since a resonator operating in the bending mode has a resonant frequency which is dependent upon its length and also upon the cross-sectional moment of inertia in the oscillation direction. The cross-sectional moment of inertia is determined by the cross-sectional area and shape and, thus, for an oscillator operating in the bending mode not only its length but also its cross-section can be selected to establish the resonant frequency. An advantageous method of coupling resonators operating in the bending mode can consist of longitudinal coupling wires which have small cross-section and such wires of small cross-section provide relatively stable coupling and it is thus possible to construct filters having relatively large band width characteristics as, for example, for use in the carrier frequency transmission field. Because of the small cross-sectional area of the longitudinal coupling wires, such filters are only slightly susceptible to so-called secondary waves, in other words, of undesired spurious oscillations of the overall filter system since such secondary waves are only slightly coupled to the individual resonators.
Mechanical filters such as described above are shown in German Pat. No. 15 41 975. This patent also discusses another problem which exists in mechanical filters which is the production of attenuation poles in the transmission characteristic of the filter. The introduction of attenuation poles can be accomplished in a known manner to achieve a rise in the gradient of the attenuation characteristic at desired frequency points which makes it possible to avoid the use of filter resonators and this is particularly desirable when the filter must be constructed so as to fit into a small space. In addition, the use of a smaller number of resonators allows a reduction in the group transit time of the signals in the pass band.
In German Pat. No. 15 41 975, at least one additional coupling element is used for the production of attenuation poles and such additional coupling element additionally connects to other resonators which directly join each other in the electrical mode of operation. The physical mode of operation of such additional coupling elements provides for the signal clamping and extinction of specific frequencies so that such frequencies will not be transmitted through the resonators. This frequency is designated the pole frequency and in known arrangements for the production of this mode of operation it is necessary to provide an additional coupling element which extends obliquely relative to the main coupling element which is the coupler which determines the band width so as to connect different sections oscillating in phase opposition of the resonators which are coupled by the additional coupling element. Oblique coupling elements of this type are fundamentally difficult to construct and install in a practical embodiment for two reasons. Additional bending components are present in such additional coupling element and cause attenuation distortions since the longitudinal oscillation is also used as a useful oscillation for the obliquely arranged additional coupling element. Furthermore, the coupling element must be attached to the resonators at points at which the bending amplitude of the resonators change considerably in dependence upon the local position and it becomes very difficult in large scale production to manufacture a plurality of such filters which have properties that are as far as possible identical to one another due to the criticality of the attaching points of the obliquely arranged coupling wires.